how can i use my phone 4s verizon to another carier

You will need to contact Verizon to get the SIM slot unlocked. Note that this works only for international use; Verizon will not unlock the iPhone to use with any other US cell carrier.

Regards.

It can not be used on any other US carrier. Ever. If you are a Verizon customer in good standing, you can get the SIM slot unlocked for international travel. If not, you're out of luck. You can't use it on anthing other than Verizon.